What exercises can I do for my curved lumbar spine so to relieve the stiffness?

Stretch. Stretch exercise can help. You can learn this type of exercises from a physical therapist, physician, trainer, yoga instructor. Be prudent. Warming up helps. If an exercise is particularly uncomfortable, do not do it. When starting any new exercise, ease into it. Some soreness is natural at the beginning, but it should not be terribly painful. Some stiffness is part of the natural aging process.
Core stabilization. I would need a lot more info, but in general a good core, trunk stabilizers, and low back muscles therapy program should help. The stronger all your muscles are surrounding the spine, the less force gets exerted across it. Stretches are important too for stiffness. I don't have enough space to type them out, but a physical therapist could help.